A new app called Canary is built to help you determine if that last bong rip before hitting the road was a bad idea, legally speaking.

Last week, NORML, a group dedicated to legalizing marijuana announced a new iPhone app designed to prevent stoned driving.

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The app, called Canary, allows users to determine whether they’re in a suitable condition to drive. It runs through a battery of tests: remembering a sequence of numbers, balancing on one foot, playing a digital whack-a-mole game, and then estimating a time period of 20 seconds. By comparing the results against a personal baseline or a collective average, users receive a green, yellow, or red light assessing their level of functioning.

Drivers can be impaired by factors as diverse as fatigue, alcohol, or legal medications, but NORML is plugging it as a way for potential drivers to determine whether they are too high to drive, which has proven to be a confounding issue as more states allow medical and recreational marijuana.

The developer is Marc Silverman, a former engineer and medical device executive. He’s not employed by NORML, but agreed to partner with the organization. Since the announcement last week, he says Canary, which costs $4.99, has been downloaded thousands of times, though of course it’s impossible to tell if it has kept anyone from driving impaired, or prevented any accidents.

We believe that the vast majority of people are responsible.

The app depends on someone who may be impaired taking the initiative to protect themselves and others. “It requires a responsible citizen to take this test, someone that cares,” Silverman says. “What we’re hoping, what we believe, is that the vast majority of people are responsible. They don’t want to hurt anybody else. They don’t want to hurt themselves.”

He initially wanted to call the app The Responsible Citizen. “No, I’m not a marketing guy,” he admits. “The idea is to cut law enforcement out for all the right reasons. We’d like people to go about their business, and drive safely, and never encounter a law enforcement official, but even more important—never encounter a pedestrian or another driver.”

Loosening medical and recreational pot laws in many states have been permitted under a 2013 Department of Justice document known as the Cole Memo, for then Deputy Attorney General James M. Cole. The memo has been widely interpreted to say that the federal government will respect state marijuana laws as long as they do not violate several tenets, including that they don’t enable distribution to minors, do not sell drugs across states lines, and do not facilitate organized crime. One of the memo’s more ambiguous requirements is: “Preventing drugged driving and the exacerbation of other public health consequences associated with marijuana use.”

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Common sense suggests that driving while high isn’t a good idea, but it’s proven difficult to ascertain how much of a problem it is or how it should be enforced. Alcohol passes through the body quickly and a driver’s blood alcohol content is a relatively clear indication of how impaired they are. In Washington State for example, it’s illegal to drive with a blood alcohol content above .08% (.02% for minors.) Registering a higher concentration at a checkpoint say, is a crime, even if the driver hasn’t been involved in an accident.

Washington state and Colorado, which have legalized marijuana for medical and recreational use, apply a similar model for marijuana. In Colorado, for example, there are guidelines suggesting that someone who has consumed 5 nanograms of THC—the main psychoactive ingredient in marijuana—per milliliter of blood is not safe to drive. Some states, meanwhile, consider lower amounts of THC in the bloodstream unsafe.

According to published research, however, it’s not clear that there is a strong correlation between a specific weed concentration and impairment, in part because marijuana acts on the body differently than alcohol. THC can turn up in urine and blood weeks after marijuana use, long after the effect has worn off. A recent Reuters article cited three parties developing marijuana “breathalyzers,” none of which could accurately measure whether the subject is in a suitable condition to drive. (None of the three parties, two companies and a professor at Washington State University agreed to comment for this article.)

Some apps designed to prevent drunk driving included similar methods to Canary often alongside calculators that could determine blood alcohol content. Thus far, there does not appear to be a equivalent, straightforward calculation for marijuana use.

Two reports released earlier this year by the Department of Transportation’s National Highway Traffic Safety Administration (NHTSA) complicate the picture further. One study found that, on weekend evenings and during business hours, almost a quarter of drivers tested positive for the presence of drugs other than alcohol, including illegal drugs like marijuana and over-the-counter and prescription drugs with the potential to weigh on driving ability such as sedatives.

The agency’s second study offers the sure-to-be-controversial conclusion that “some of [the available research has] suggested that marijuana use has minimal or no effect on the likelihood of crash involvement, while others have estimated a small increase in the risk of crash involvement.” The study found that THC, the main psychoactive ingredient in marijuana, did increase the risk of accidents but after accounting for age and sex—young men get in more accidents—it suggested that the increased odds of an accident might be attributable to demographics rather than the drug. Combining alcohol and drug use before driving is roughly as risky as just drinking and driving, which is to say very risky.

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An agency spokesperson writes that, “A number of studies have documented the observation that drivers under the influence of cannabis tend to drive slower or more cautiously than sober drivers,” but that more research is needed to factor in other effects. Using similar reasoning, pro-cannabis activists sometimes say that people who are high are also more likely to just stay home.

While it appears impossible to enforce, Canary could potentially take a number of impaired drivers off the roads, which would be keeping with NORML’s longstanding goal to distance marijuana use and law enforcement. Invoking the NHTSA study, Paul Armentano, NORML’s deputy director, pointed out that other common conditions also elevate the risk of driving without attracting attention from authorities. “No one is recommending we accept some sort of criminal prohibitions against driving with more than two people in the car or we say that pregnant woman shouldn’t be allowed to drive until they have their child because it’s a traffic safety threat,” he says. “It would be absurd if anyone proposed these sort of things.”